[heap] Put print behind flag

Bug: chromium:863362
Change-Id: I88896d7477d893f1b7fae08f6dfd5709748a6edd
Reviewed-on: https://chromium-review.googlesource.com/1138080Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54461}

src/heap/gc-tracer.cc

@@ -277,9 +277,11 @@ void GCTracer::ResetIncrementalMarkingCounters() {
 void GCTracer::Stop(GarbageCollector collector) {
   start_counter_--;
   if (start_counter_ != 0) {
-    heap_->isolate()->PrintWithTimestamp("[Finished reentrant %s during %s.]\n",
-                                         Heap::CollectorName(collector),
-                                         current_.TypeName(false));
+    if (FLAG_trace_gc_verbose) {
+      heap_->isolate()->PrintWithTimestamp(
+          "[Finished reentrant %s during %s.]\n",
+          Heap::CollectorName(collector), current_.TypeName(false));
+    }
     return;
   }